Exemplary embodiments relate to a method of programming a nonvolatile memory device and, more particularly, to a method of programming a nonvolatile memory device, which is capable of preventing a reduction in the reliability of a program operation resulting from interference between neighboring memory cells.
With an increase in the degree of integration of nonvolatile memory devices, the interval between memory cells for storing data is gradually narrowed. If the interval between the memory cells is narrowed, interference between the memory cells may increase during program, erase, and read operations of the nonvolatile memory device. In particular, during the program operations, interference between the memory cells becomes significant.
FIG. 1 is a circuit diagram showing part of a nonvolatile memory device, and FIG. 2 is a diagram illustrating a related phenomenon.
Referring to FIG. 1, the memory cell array of the nonvolatile memory device includes a plurality of cell strings Ev and Od. Each of the cell strings Ev and Od includes a drain select transistor DST and a source select transistor SST and a plurality of memory cells F0 to Fn coupled in series between the drain select transistor DST and the source select transistor SST.
In particular, the even cell string Ev and the odd cell string Od are alternately selected in order to prevent/reduce interference occurring during the program operations. For example, if the even cell string Ev is selected, the odd cell string Od is not selected.
However, with an increased integration of semiconductor devices, although the even cell string Ev and the odd cell string Od are alternately selected and a program operation is performed on a selected cell string, interference may occur during the program operations of neighboring cells.
Referring to FIGS. 1 and 2, the threshold voltage of an even memory cell Ce is increased to a target level by programming the even memory cell Ce. Here, the even memory cell Ce may have an erase state ER, a first program state PV1, a second program state PV2, or a third program state PV3 depending on the level of the threshold voltage. In the case in which the even memory cell Ce has been programmed to have the first program state PV1, during the programming of an odd memory cell Co adjacent to the even memory cell Ce, the threshold voltage of the even memory cell Ce may rise because of interference between neighboring memory cells.
Due to such interference, threshold voltages may vary from target threshold voltages.
If, as described above, the threshold voltages of previously programmed cells rise due to interference generated during the program operations of neighboring cells, the reliability of a nonvolatile memory device may be reduced.
In order to alleviate such a concern, when selected memory cells are read, a read voltage may be controlled depending on whether neighboring cells have been programmed. Use of such a method, however, may slow the operation speed. In particular, an algorithm to determine whether a program has been performed and perform a re-program based on a result of the determination may be relatively complicated.